Wood is a material fit for the 21st century
When we think of wood, the first thing that comes to mind is probably furniture. In some countries, like the US, small houses are often still made of timber. But scientists and architects around the world are re-thinking this material, improving its properties and expanding its range of application, in the hope to develop an environmentally friendly substitute to the performant, but energy intensive, engineered materials we use today.
The structure of wood
We are all familiar with wood, but what exactly is it? Well, a simple definition is to say that wood is what trees' trunks are made of. Not all plants grow a woody trunk; we call trees those that do, and they make this sturdy brown material to carry nutrients from the roots to the tips of the tree and to supports the tree’s structure against the weather.
Wood has a cellular structure: if you looked at it under a microscope, you would see a bundle of fibers running along the trunk as well as other types of cells going in other directions. Three main ingredients make up the structure of wood: cellulose, lignin and hemicellulose. Cellulose is a chemical made out of sugar molecules and makes fibres that are very strong in tension. These fibres are like chains - which means if you pull on them they will resist but if you try to squash them, they will crumple. This would be very bad news for the tree, so to prevent it, trees strengthen their wood with lignin, a stiff polymer with a messy structure that stops the cellulose chains from collapsing under their own weight. Lignin is also what gives wood its brown colour. Finally, to stick the two together, there’s hemicellulose, which gives wood flexibility.
Deciduous trees like beech and oak produce what we call hardwood, whereas coniferous trees like spruce and pine make softwoods. These two groups of trees have been evolving separately for hundreds of millions of years so their woods have very different structures, with hardwoods being a lot more complex. However, just to confuse things, not all harwoods are hard and not all softwoods are soft: balsa wood, which is a very soft wood used to make model aeroplanes, is technically a hardwood - whereas yew trees, which you often see in graveyards, give a very hard wood but are categorised as softwoods.
Although it may look sturdy, at least half of a tree is made from the air. Thanks to photosynthesis, trees transform carbon dioxide from the atmosphere into the sugars that become the wood. To grow one kilogram of wood a tree has to draw down 1.6 kilograms of CO2. That’s why everyone talks about trees as one of the main solutions to oppose climate change.
Tall buildings made of timber
Small houses around the world are still made of wood today, but to make the skyscrapers or tall buildings one sees in cities, engineers use steel and concrete. These materials form the very foundation of our modern civilisation, but they require a lot of energy to produce and lead to massive amounts of carbon emissions, which drive global warming. This is why architects and engineers are attempting to develop new types of wood to make tall buildings.
One special material is called cross-laminated timber (CLT) and consists of wood planks glued together in mutually perpendicular directions to give uniform and high strength. This and other engineered wood materials are being used to build the Black & White building in London, which is set to become the tallest timber office building in the whole capital when complete in 2022. CLT is strong, it’s safe against fire and, if kept well, it is not going to rot, making the building last for at least 60 years. Moreover, building with timber is a lot simpler and safer, because the structures are pre-built in a factory and carried to the construction site. This makes building much faster, too, according to Waugh Thistleton Architects, the firm that designed the Black & White building. Wood also has much better acoustic properties than steel and concrete, so you don’t need to use additional material to make rooms comfortable for the people using them.
Sequestering carbon in buildings
An advantage of building with timber is that its carbon footprint is a lot lower, according to researcher Will Hawkins from the University of Bath. Steel and concrete must be produced at very high temperatures, which requires energy. Moreover, the chemical processes that take place when these materials are made intrinsically lead to carbon emissions which cannot be avoided. Instead, the processes that transform the trunk of a tree into a wooden plank require much less energy and could potentially be electrified in the future, leading to additional energy savings.
But the best aspect of wood is that it comes from trees, and while trees grow they draw down carbon dioxide from the atmosphere, helping oppose climate change. Storing this carbon in buildings is a good idea because they are the man-made objects that last the longest. However, we should be careful to design buildings as efficiently as possible, because trees are also important for biodiversity in forests. Moreover, the need for land to grow food or biomass for a growing population is high, so we cannot devote too much land to growing trees for timber. All these needs must be balanced to achieve a sustainable long-term solution.
Superwood and transparent wood
Scientists and engineers are also modifying wood to improve or change its properties. A research group at the University of Maryland recently succeeded in making transparent wood, by removing the lignin and substituting it with an optically transparent resin. This transparent wood material has better thermal properties than glass, is cheaper and more environmentally friendly to produce.
Teng Li, a researcher at the same university, also managed to compress and densify wood to create a material as strong as titanium and as hard as stainless steel. This involves two steps: firstly, a chemical process to remove the lignin from the wood, and then a pressing process which compacts the wood. Using this technique, the researchers made a wooden knife that can cut through steak!
The most useful wood ingredient is cellulose, and scientists are finding ways to extract cellulose from wood to make materials with very special properties. Silvia Vignolini, a researcher in the Department of Chemistry at the University of Cambridge, exploits the properties of cellulose at the nanoscale to make colourful, biodegradable pigments that can be used in cosmetics.
Cellulose has an ordered, crystalline structure which interacts with light in very special ways: it bends light rays so that we see colours such as blue, green and red. By controlling the chemistry of a solution with cellulose nanocrystals inside, the researchers can tune the colour of the cellulose. This has the potential to substitute plastic-based components of hundreds of cosmetic products in the industry, thus making for a sustainable, biodegradable solution.
Overall, we have only begun to scratch the surface of the immense potential wood has to solve the most pressing challenges of the 21st century. By looking at this old material in new ways, we can see how nature is offering us one of the best solutions to restore harmony with the environment.